允许参考寿命比关闭寿命更长

Question

I'm getting to grips with Rust by trying to implement a singly-linked list. Here are my data structures:

struct Linked<T> {
    head: Option<Box<Node<T>>>
}

struct Node<T> {
    data: T,
    next: Option<Box<Node<T>>>
}

Now, I'd like to add an iterator to this:

struct LinkedIter<'a, T: 'a> {
    node: Option<&'a Node<T>>,
}

I've written a .iter() method for Linked<T> , which compiles and works fine.

impl<T> Linked<T> {
    fn iter(&self) -> LinkedIter<T> {
        LinkedIter { node: match self.head {
                Some(ref node) => Some(&**node),
                None           => None
            }
        }
    }
}

Now, this match block is converting an Option<Box<Linked<T>>> to an Option<&Linked<T>> . This is exactly what the Option::map() method is for. So I reimplemented this method, using head.as_ref() instead of head to avoid taking ownership of the Option 's contents in the closure.

impl<T> Linked<T> {
    fn iter(&self) -> LinkedIter<T> {
        LinkedIter { node:
            self.head.as_ref().map(|b: &Box<Node<T>>| &**b)
        }
    }
}

Unfortunately, the reference created in the closure cannot be allowed to outlive the closure, because it refers to something passed into the closure as a parameter. The compiler complains (paraphrased a bit):

error: cannot infer an appropriate lifetime for borrow expression due to conflicting requirements:

first, the lifetime cannot outlive the anonymous lifetime #1 defined [in the closure]

but, the lifetime must be valid for [the lifetime of the iter function]

so that the [ node: ] expression is assignable

How can I explain to the compiler that the reference will still be valid after the closure ends?

(Playground)

Answer 1

The idea is to inform the compiler that the reference you're taking in map() lives as long as the iterator. I got it working like that:

impl<T> Linked<T> {
    fn iter<'a>(&'a self) -> LinkedIter<T> {
        LinkedIter { node:
            self.head.as_ref().map(|b: &'a Box<Node<T>>| &**b)
        }
    }
}